Composite article and method of making same

ABSTRACT

A method of making a composite article and a composite article specifically adapted for use in high temperature, corrosive and errosive environments comprising a carbon fibrous substrate, including a pyrolytic carbon sheath formed about each fiber of the substrate; a metallic carbide, oxide, or nitride compliant coating over the coated fibers of the substrate; and an impermeable metallic carbide, oxide or nitride outer protective layer formed about the entire periphery of the coated substrate. In accordance with the method of the invention, the compliant metallic coating is applied to the fibers in a manner such that any mechanical stresses built-up in the substrate due to a mismatch between the coefficient of thermal expansion of the fibrous substrate and the coating are effectively accommodated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to carbon or ceramic-metalliccomposite articles and the method of making same. More particularly theinvention relates to unique carbon-silicon composite articles for use inhigh temperature, hostile fluid environments.

2. Discussion of the Prior Art

During recent years considerable effort has been directed toward thesearch for structural materials having increasingly high temperaturecapabilities, and superior dimensional stability, corrosion resistance,erosion resistance and tolerance to damage. In this connectionsubstantial work has been done with metals, monolithic ceramics andcarbon-graphite materials. Metals display toughness and tolerance todamage but are relatively limited in their temperature capability.Monolithic ceramics can withstand high temperatures and oxidizingenvironments, but are vulnerable to structural damage. Carbon-graphitecomposites, on the other hand, can withstand high temperature andstructural damage but are subject to oxidative degredation. In view ofthese facts the development of new composite materials has commandedconsiderable attention. Since composites can combine many of theattractive features of metals while ameliorating various of thestructural and degradation problems associated with carbons andceramics, they are ideally suited for very high temperature, hostileenvironment applications.

In pursuit of tough high temperature composites, various types ofcoating processes have been suggested. These processes generally involvecontacting a molten metal and a carbon body under certain conditions,generally for the purpose of producing protective coatings for thecarbon body in its environment of intended use. For example, Smiley U.S.Pat. No. 3,019,128 discloses applying molten metal to a carbon body toform a metal carbide surface layer, which in combination with metal andmetal oxide layers, produces a refractory and heat transfer coatingdesirable on rocket nozzles and the like.

Similarly, Gurinsky U.S. Pat. No. 2,910,379 discloses a process in whichmolten metal is applied to a carbon liquid nuclear fuel container toprevent deleterious poisoning arising from graphite reaction withnuclear fuels and fission products.

Other coating disclosures involving molten metal-carbon body contact areSteinburg U.S. Pat. No. 2,929,741, Winter U.S. Pat. No. 2,597,964 andAcheson U.S. Pat. No. 895,531.

The U.S. Pat. to Fatzer et al, No. 3,925,577 describes a process forproducing coated isotropic graphite members wherein a layer of siliconis first deposited on a graphite body and then the body is heated to atemperature to cause the silicon to melt and penetrate the pores of thegraphite. Finally the article is coated with a layer of silicon carbide.In Hacke U.S. Pat. No. 3,348,967 a somewhat similar process is describedin which graphite or charcoal bodies are impregnated with a molten metalwhich will react therewith to form carbide, thereby enabling theproduction of a wide variety of useful products. As will become clearfrom the discussion which follows, these prior art patents whilegenerally related to the present invention are clearly distinguishabletherefrom.

A common thread running through many of the prior art disclosuresconcerning metallic coating of carbonaceous materials, and one whichserves to clearly distinguish the present invention, has been theheretofore unquestioned acceptance of the basic premise that thesubstrate material should be isotropic and that it must have anexpansion coefficient approximating that of the coating. This has beentraditionally believed necessary to prevent cracking and spalling of thecoating due to stresses induced by differences in the expansioncoefficients when the article is subjected to thermal cycling. The U.S.Pat. No. to Howard et al 3,393,085 discusses this premise in somedetail.

Another well established prior art premise was that the metalliccoatings should have an adherent mechanical or chemical bond to thesubstrate material to assure proper load transfer and to guarantee thestructural integrity of the coated composite system. As will bediscussed in greater detail hereinafter, Applicant has also found thislatter requirement to be not only unnecessary, but, in fact undesirablein the practice of his invention.

By way of example and to illustrate the aforementioned prior artconcepts, graphite susceptions have long been used to heat siliconwafers for semiconductor processing. Because graphite is very porous andprovides a means of entrapping undesirable gases and other contaminants,the susceptors are typically coated with a chemical vapor deposited(CVD) silicon carbide to render them impermeable and non-reactive.Because silicon carbide has a high coefficient of thermal expansion(CTE) of on the order of 4.5 to 5.0 in/in/°C., a high expansion 4.2in/in/°C. nominal CTE graphite is used as the substrate material toassure an economic susceptor life. It is well known, however, that theactual characteristics exhibited by graphite materials can vary 10 to15% from the nominal values described in the literature. Accordingly,given substrate characteristics, including CTE characteristics, varywidely from lot to lot. As a result of these variations in substrateexpansion coefficients, the economic life of the susceptors is quiteunpredictable. Compounding the problem is the fact that coating life isalso highly variable and directly relates the matching of coating andsubstrate expansion coefficients. Thus, current practice by susceptormanufacturers is to guarantee susceptors for not more than four completetemperature cycles.

In a similar vein, various prior patents U.S. Pat. including Nos.2,512,230 and 1,948,382 describe composites comprising coatings ofsilicon carbide on monolythic and composite carbonaceous substrates toprovide erosion protection for the substrate as well as interlayersupport and bonding between the substrate and coating. In suchapplications it has been uniformly taught that the thermal expansioncoefficients of the substrate should approximate that of the interlayerof coating if cracking or spalling of the interlayer coating is to beprevented.

For the reasons previously discussed, great difficulty has beenexperienced in satisfactorily and economically manufacturing compositearticles suitable for very high temperature applications in which thecoefficient of thermal expansion of the coating and the substrate ismatched and in which the coating satisfactorily adheres to the basematerial.

As will be appreciated from the discussion which follows, the process ofthe present invention totally overcomes the prior art problems ofcoating adherency and CTE matching and provides a unique CTE mismatchedcomposite article which will maintain its dimensional stability and willeffectively resist corrosion even in hostile environments at very hightemperatures.

In addition to the previously identified prior art patents, applicant isfamiliar with the following U.S. Pat. Nos. which serve to vividlyillustrate the high degree of novelty of the present invention:3,914,508, 3,762,644, 3,759,353, 3,676,179, 3,673,051, 3,275,467,2,614,947, 1,948,382.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a structurallyunique composite article consisting of a multiphase system comprising acarbon fiberous substrate, a metallic carbide, oxide or nitridecompliant coating over the fibers of the substrate and an impermeablemetallic carbide, oxide or nitride outer protective layer or seal coatformed about the periphery of the coated substrate.

It is another object of the invention to provide a composite article ofthe aforementioned character which will substantially maintain itsdimensional stability and strength even under severe high temperatureoxidizing conditions.

It is another object of the invention to provide a composite article ofthe class described which is substantially resistant to corrosion anderosion by high temperature, hostile gas, particulate and fluidenvironments.

It is a particularly important object of the present invention toprovide a composite article of the character described in the preceedingparagraphs in which the aforementioned metallic carbide, oxide ornitride compliant coating is controllably applied to the carbon fibroussubstrate in a manner such that any mechanical stresses built up in thesubstrate due to mismatches in coefficients of thermal expansion betweenthe fiberous substrate and the coatings are effectively accomodated orrelieved.

It is a further important object of the present invention to provide aprocess for making composite articles of the character described in thepreceeding paragraphs in which the metallic carbide, oxide or nitridecompliant coating is controllably applied to the carbon fibroussubstrate in a manner such that the individual fibers of the substrateare free to move relative to the applied coating.

More particularly, it is an object of the invention to provide a processas described in the previous paragraph in which a pyrolytic carboncoating is first deposited by chemical vapor deposition (CVD) about eachof the fibers in such a manner that each fiber is substantially encasedin a non-adherent pyrolytic carbon casing and then a metallic carbide,oxide or nitride coating is applied over the coated fibers in such amanner that the fibers remain freely movable relative to the appliedcoatings.

It is another object of the invention to provide a composite article asdescribed in the previous paragraph in which each fiber of the substrateis encased in a uniform CVD type carbon casing to promote superior loadtransfer from fiber to fiber when the article is stressed. This CVDcarbon casing also provides a mechanical interface for increasing thesurface fracture energy of the composite structure thus resultingsubstantial toughness and flaw resistance.

It is still another object of the invention to provide a process of theaforementioned character in which, following the coating of the fibers,an impermeable carbide, nitride or oxide coating is controllably formedabout the entire periphery of the substrate to seal it against hostileenvironments.

In summary, these and other objects of the invention are realized by acomposite article produced by a method comprising the steps of forming astarting substrate from a multiplicity of carbon fibers selected from agroup consisting of pyrolyzed wool, rayon, polyacrylonitrile and pitchfibers; suspending the starting substrate within a first controlledenvironment; forming an intermediate substrate by heating the startingsubstrate to a temperature of between approximately 1500° F. andapproximately 2200° F. while exposing the starting substrate to ahydrocarbon gas to form a uniform layer of pyrolytic carbon about eachof the fibers in the starting substrate; removing the intermediatesubstrate from said first controlled environment and forming it into ashaped substrate having the approximate shape desired of the end productcomposite article; supporting the shaped substrate in a secondcontrolled environment while heating it to a temperature of betweenapproximately 1800° F. and approximately 3200° F.; and forming adiffusion coated article by reacting the shaped substrate with asiliceous material for a period of time sufficient to permit the siliconto react with the pyrolytic carbon coating deposited on the fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reproduction of a photomicrograph (70×) showing theappearance of the coated fibrous substrate of the invention prior to theapplication of the final seal coat.

FIG. 2 is a reproduction of a photomicrograph (1500×) illustrating theappearance of the coated fibers of the carbonaceous fibrous substrate.

FIG. 3 is similar to FIG. 2 showing a reproduction of a photomicrograph(2200×) of the coated fibers.

FIG. 4 is a side elevational view, partly in section showing a testsection of a net dimension turbine augmentor divergent flap manufacturedin accordance with the method of the present invention.

FIG. 5 is a greatly enlarged fragmentary cross-sectional view of thearea designated in FIG. 4 as "5" illustrating diagramatically the natureof the internal configuration of the test section.

DISCUSSION OF THE PREFERRED EMBODIMENTS

Before proceeding with a detailed discussion of the preferredembodiments of the present invention, the following definitions of thetechnical terms used herein are presented to facilitate a clearunderstanding of the nature and scope of the invention:

1. Composite product--a product comprising a carbon, graphite or ceramicsubstrate and one or more metallic carbide, oxide or nitride coatingsover the substrate material.

2. Starting or basic substrate--as used herein, the starting substrateor interim product shape before the application of a metallic coating.

3. Carbon fibrous substrate--a starting substrate comprising carbonmaterial in fibrous form.

4. Fiber volume--volume of carbon fibers present in the given substrate.

5. Non-woven--coherent fibrous material formed without interlacing ofthreads, such as batting or felt.

6. Woven--fabric formed by interlacing warp and filling threads on aloom, or the like.

7. Ceramic--metallic or other inorganic oxides generally classed asglass-forming oxides.

8. Pyrolytic or "CVD" material--a material made from the thermaldecomposition of a gas containing the material.

Stated in simple terms, the composite article of the invention consistsof a three or more phase system comprising a basic substrate of carbonfibrous and/or ceramic materials, a metallic carbide, oxide or nitridecompliant layer over the substrate and an impermeable carbide, oxide ornitride protective layer formed over the entire periphery of the coatedsubstrate. An important feature of the article is the absence of astrong bond between the fibers and the matrix system to accommodate themismatch in expansion coefficient between the fibrous substrate and thecarbide, oxide or nitride compliant and protective layers.

The articles of the invention are well suited for a variety ofapplications including turbine rotors, turbine augmentor divergent flapsand Diesel engine pre-combustion chambers made up of a carbon fiber,carbon (resin and/or pitch char and/or CVD deposit) matrix componentwhich has been preformed to a selected component net geometry. Each ofthe substrate fibers is encased in a non-adherent, uniform, CVD typecarbon case so as to promote good load transfer from fiber to fiber whenthe article is stressed. This also provides a mechanical interface forincreasing the surface fracture energy of the composite structure, thusresulting in greater toughness and flaw resistance.

Because carbon/carbon composites actively react with oxygen when heatedto temperatures in excess of 300° C., the outer portions of the articleare reacted with silicon to form a continuous silicon carbide casearound each of the fibers. This treatment effectively imparts a poroussilicon carbide shell about the periphery of the selected carbonsubstrate geometry. The expansion coefficient of this shell isessentially that of monolithic silicon carbide, approximately two timesthat of the carbon preform. In practice, a mis-match in expansioncoefficients of only a few percent between the substrate and the coatingor outer shell is sufficient to induce mechanical stresses which causecracking and ultimate failure of the protective layer or oxidation ofthe substrate. The lack of bond between the fiber and the carbon andsilicon carbide case, however, allows the carbon fibers and siliconcarbide case to move independent of one another on a microscopic scale,thus providing a compliance mechanism that effectively reduces thermallyinduced mechanical stresses. The appearance of the coated fibers isclearly illustrated in FIGS. 1 and 2 of the drawings.

The article in this interim state with its compliant outer shell ofsilicon carbide remains quite porous. Therefore, to provide completeprotection for use in high temperature oxidizing environments, asubsequent infiltration of the porous compliant layer with impermeableCVD silicon carbide is applied to seal the surface with a meterial whoseexpansion coefficient is compatible with that of the compliant layer.The final article thus formed is remarkably stable and highly corrosionresistant even in extreme environments.

Referring breifly to FIGS. 4 and 5 of the drawings, one form of articlemade by the method of the invention is there illustrated. This article,which is a test section of a turbine augmentor divergent flap, comprisesa central section 12 made up of carbon felt, chopped fiber or maceratedmaterial. At each end of the test section is a solid end portion 14 madefrom a carbon/carbon high strength material, such as is commerciallyavailable from the Hitco Division of Arco. Each end section 14 isprovided with a bore 16 adapted to receive a pivot pin or rod.

Surrounding the central section 12 and end portions 14 is a materiallayer 18 comprising a woven carbon fibrous material such as carbon orgraphite cloth commercially available from The Union Carbide Company andothers. Formed about the article thus constituted, and in conjunctiontherewith comprising the starting substrate of this embodiment of theinvention, is a carbon fibrous material generally designated by thenumeral 20.

In a manner which will be described in greater detail in the paragraphswhich follow, the individual fibers of the material 20 are coated with alayer, or sheath, of CVD carbon and then with a compliant layer ofsilicon carbide. Finally, the entire surface of the test specimen issealed with an outer layer 22 of CVD silicon carbide which extends aboutthe entire periphery of the article.

Referring to FIG. 1, the character of the coated fibrous material 20 isthere vividly illustrated. FIG. 1 is a reproduction of a photomicrographof the coated fibrous material at 70 times magnification. Turning toFIG. 2, which is a reproduction of a photomicrograph at 1500 timesmagnification, the fiber, the CVD coating about the fiber and thesilicon carbide coating superimposed thereupon, are clearly visible. InFIG. 3, which is a reproduction of a photomicrograph taken at 2200 timesmagnification, the ends of the individual fibers can clearly be seenprotruding from the CVD carbon coating and the compliant coating ofsilicon carbide. As will be appreciated from the examples which follow,the compliant silicon carbide coating is applied to the fibers in amanner such that any mechanical stresses built up in the substrate dueto a mismatch between the coefficient of thermal expansion of thefibrous substrate and the coating are effectively accommodated. Thisimportant feature of the present invention is clearly illustrated inFIGS. 2 and 3 of the drawings.

It is to be appreciated that the article shown in FIGS. 4 and 5 of thedrawings is merely exemplary of the type of products which can be madein accordance with the method of the present invention. Other highlyuseful products of the invention include turbine rotors, deisel enginecombustion chambers, and numerous specially designed products fornuclear and aerospace applications.

As will be clearly illustrated by the examples which follow, the methodof the invention stated in simple terms comprises the following steps:First, a multiplicity of carbon fibers such as rayon fibers areassembled into a basic or starting substrate. Next the startingsubstrate is placed in a controlled environment, heated to between about1500° F. and 4200° F. and exposed to a carbonaceous gas such as methane.During this step a uniform layer of CVD carbon is deposited about eachof the fibers of the substrate. Following this step the interimsubstrate thus formed is machined or otherwise formed into theapproximate shape of the end product. Next the shaped substrate formedin the previously described step is placed in a second controlledenvironment and heated to about 1800° F. to about 3200° F. The heatedshaped substrate is then reacted with a siliceous material for a periodof time sufficient to permit the silicon to react with the CVD carboncoating deposited on the fibers. Finally, the article thus formed isonce again heated in a controlled environment and exposed to a gascontaining carbon and silicon such as trichlorosilane. This step forms auniform CVD seal coating of silicon carbide about the entire peripheryof the article.

EXAMPLE NO. 1

Using carbonized rayon felt as a starting material, a starting substratewas constructed. In this instance the starting substrate wasapproximately 4 inches wide, 8 inches long and about 1 inch thick. Thedensity of the substrate was on the order of 0.1 gm/cc and the fibervolume was about 15%. Next, the starting substrate, along with severalcontrol specimens, was placed into a first controlled environment, whichin this case was a vacuum chemical vapor deposition furnace ofconventional design. The temperature of the substrate was then raised toabout 1800° F. while a vacuum of on the order of 15 mm Hg was maintainedwithin the deposition chamber. A secondary, or intermediate, substratewas formed by controllably flowing methane gas interstitially of thesubstrate for a period of time of about 50 hours. Through this techniquea uniform layer of pyrolytic carbon was deposited about each of thefibers within the substrate to form an intermediate substrate having adensity of on the order of 0.75 gm/cc.

Following the aforementioned infiltration step, the intermediatesubstrate was cooled, removed from the vacuum furnace and transferred toa machining area. In the machining area the intermediate substrate wasmachined in a conventional manner to form a shaped substrate. In thisinstance the shaped substrate was constructed in the configuration of anet dimension turbine augmentor divergent flap test sectionapproximately 6 inches wide by 6 inches long by 1/2 inch thick (see FIG.1).

Following the machining step, the divergent flap section, or shapedsubstrate was supported in a vacuum chamber, or second controlledenvironment directly above a crucible containing molten elementalsilicon. The shaped substrate was then heated to about 2700° F. in amild vacuum. With the substrate at this elevated temperature it waslowered into the molten silicon and maintained totally immersed forabout four minutes. Following immersion the diffusion coated substratewas withdrawn from the molten silicon and maintained at temperature in aposition directly above the silicon crucible for a period of time ofabout two minutes.

Next, the diffusion coated article thus formed was cooled, removed fromthe vacuum chamber and transferred to an inspection area. Followingremoval of the excess silicon by means of an etching process, precisiondimensional inspection was performed on the part. This inspectionrevealed that the diffusion coated article exhibited the dimensions ofthe shaped substrate within plus or minus 0.002 inches. Visualinspection of the control specimens which had been similarly processedshowed the carbon fibers to be essentially unaffected by the methane orsilicon treatment. Only the ends of the fibers exposed by the machiningshowed any signs of reaction with the silicon. Importantly, no bond wasfound to exist between the carbon fibers and the silicon carbide coatingformed by reacting the shaped substrate with the molten silicon. On theother hand, a reaction with the CVD carbon deposited on the fibersduring the preliminary methane treatment was most evident. Thisreaction, however, was confined to the CVD carbon sheath surroundingeach of the fibers. In all cases no chemical or diffusion bonds wereobserved to exist betweed the fiber and/or matrix system. Accordingly,the fibers were free to move at a different rate from the carbon and/orsilicon carbide and silicon matrix systems. This highly novel andimportant feature of the diffusion coated article of the presentinvention effectively minimizes any residual stresses tending to occurwithin the article. By comparison, a graphite body processed in acomparable fashion to that just described would exhibit significantinternal residual stresses. These stresses result from an inherentmismatch in the thermal expansion coefficient (CTE) between the graphitearticle itself and the coatings applied thereto. The effect of thesestresses are often catastrophic causing article cracking, crazing and/orspalling of the coatings.

Following dimensional inspection of the diffusion coated article, it wasreturned to the first controlled environment, or vacuum chemical vapordeposition furnace. Once in place within the CVD apparatus, the articlewas heated to about 2200° F. and a gas containingdimethyl-dichlorosilane was controllably passed over and about thearticle. Due to the porous nature of the diffusion coated article formedby the novel method of the present invention, a uniform coating of CVDsilicon carbide was deposited over the coated fibers of the article andabout the periphery thereof. This step provided an impermeable skin ofsilicon carbide over the entire diffusion coated article rendering itvirtually impervious to corrosion and erosion caused by hightemperatures and exposure to hostile gas and fluid environments.Subsequent testing and evaluation of the two phase compliant layer typecoated article thus formed under extremely hostile environments showedit to be highly stable and remarkably resistant to thermally inducedcracking, crazing or spalling.

EXAMPLE NO. 2

In constructing the starting substrate of this example chopped fibers ofcarbonized polyacrylonitrile were used. This substrate was also about 4inches wide, 8 inches long and about 1 inch think. The fiber volume ofthe substrate was on the order of 35%.

The starting substrate was placed into a vacuum furnace and the fibersthereof coated with pyrolytic carbon in the manner described in ExampleNo. 1. However, propane was used in lieu of methane as the carbonaceousgas.

After infiltration the substrate was removed from the CVD furnace andwas machined into a shaped substrate in the manner of Example No. 1.

Following machining, the intermediate substrate was heated to about3200° F. in a second controlled environment maintained at slightlygreater than atmospheric pressure and a slurry of elemental silicon wasdeposited on the pyrolytic carbon coated fibers.

The silicon coated article thus formed was dimensionally inspected andreturned to the vacuum furnace wherein it was once again heated to about3200° F. While being maintained at this elevated temperature a gascontaining carbon and silicon, as for example trimethyl-chlorosilane waspassed over and about the article to deposit a uniform coating ofsilicon nitride over the coated fibers and about the periphery of thearticle.

Rigorous testing of the article showed it to be dimensionally stable andresistant to cracking, crazing and spalling even in hostile gasenvironments and at high temperatures.

EXAMPLE NO. 3

A carbonized rayon cloth made up of interwoven carbon fibers was cutinto circular shaped pieces having a diameter of about 4 inches. A discshaped starting substrate was constructed by stacking a plurality of thecircular shaped pieces onto a base plate of a compression fixture. Eachlayer of cloth was rotated slightly with respect to the preceeding layerand a top plate was placed over the assembly and bolted to the baseplate. The assembly was then compressed to bring the cloth layers intointimate contact. The starting substrate thus formed exhibited a fibervolume of about 35% and a fiber density of about 1.5 gm/cc.

Next the starting substrate, along with the compression fixture, wasplaced into a CVD vacuum furnace and in the manner previously described,pyrolytic carbon was uniformly deposited over each of the fiberscomprising the disc shaped starting substrate.

The intermediate substrate thus formed was removed from the compressionfixture and machined to form a disc about 31/2 inches in diameter andabout 1 inch thick.

Following machining, the substrate was returned to the CVD vacuumfurnace and heated to a temperature of about 2200° F. A gas containingdichlorosilane was passed over and about the shaped substrate for aperiod of time of about 3 hours to form a diffusion coated article inwhich a silicon coating was formed about each of the coated fibers ofthe intermediate substrate. The temperature of the substrate was thenraised to about 2700° F. to cause a reaction between the silicon andpyrolytic carbon to form silicon carbide.

After undergoing another dimensional inspection, the still porous,diffusion coated article was returned to the vacuum furnace for finalcoating with silicon carbide in the manner described in Example No. 1.Once again the two phase, compliant layer coated article thus formedexhibited remarkable stability and durability during severeenvironmental testing.

EXAMPLE NO. 4

Using a tape material made up of closely woven, carbonized PAN fibers, acylindrical shaped starting substrate was constructed by wrapping thetape about a mandrel. This substrate exhibited density of about 0.83gm/cc and a fiber volume of about 40%.

The substrate was removed from the mandrel and placed into a vacuumfurnace wherein pyrolytic carbon was deposited on the fibers thereof inthe manner described in Example No. 1, but using acetylene as the feedgas.

Next the intermediate substrate was machined and then reacted with asiliceous material as in Example No. 1 to form a diffusion coatedsubstrate.

Finally the diffusion coated article thus formed was coated with a sealcoat of silicon carbide by heating it under vacuum to a temperature ofon the order of 1800° F. and exposing it to a gas containing carbon andsilicon as for example trimethyl-chlorosilane. This final step formed auniform CVD coating of silicon carbide over the coated fibers and theperiphery of the article rendering it virtually impervious to corrosionand erosion caused by high temperatures and exposure to hostile fluids.

EXAMPLE NO. 5

In constructing the starting substrate of this example the startingmaterial used was a macerated material comprising a multiplicity ofrandomly oriented pyrolyzed wool fibers. This starting material wasformed into a substrate which was approximately 4 inches wide, 8 incheslong and about 1 inch thick, and exhibited a fiber density of on theorder of 35%. The starting substrate was processed in the mannerdescribed in Example No. 1 except that silicon tetrachloride was used inapplying the final coating to the diffusion coated article.

EXAMPLE NO. 6

Using a macerated material having a multiplicity of chopped pitch fibersa starting substrate was constructed as in Example No. 1. This startingsubstrate which exhibited a fiber volume of about 60% was also processedas described in Example No. 1 save that silicon dibromide was used inthe final coating step.

EXAMPLE NO. 7

Using a starting substrate constructed and processed in the mannerdescribed in Example No. 3, a diffusion coating was applied by placingthe substrate in a pack containing granular silicon carbide, aluminiaand silicon. The pack and substrate were then slowly raised intemperature to about 3200° F. over a five day period to form a diffusioncoating on the substrate material. A final, or seal coating was appliedabout the periphery of the coated substrate in the manner described inExample No. 1.

Having now described the invention in detail in accordance with therequirements of the patent statutes, those skilled in this art will haveno difficulty in making changes and modifications in the individualparts or their relative assembly in order to meet specific requirementsor conditions. Such changes and modifications may be made withoutdeparting from the scope and spirit of the invention, as set forth inthe following claims.

I claim:
 1. A method of making a composite article comprising the stepsof:(a) forming a starting substrate from a multiplicity of carbon fibersselected from a group consisting of pyrolyzed wool, rayon,polyacrylonitrile and pitch fibers; (b) suspending said startingsubstrate within a first vacuum pressure controlled environment; (c)forming an intermediate substrate by heating said starting substrate toa temperature of between approximately 1500° F. and approximately 4200°F. while exposing said starting substrate to a hydrocarbon gas to form auniform layer of pyrolytic carbon about each of the fibers in saidstarting substrate; (d) removing said intermediate substrate from saidfirst controlled environment and forming it into a shaped substratehaving the approximate shape desired of the end product compositearticle; (e) supporting said shaped substrate in a second controlledenvironment while heating said shaped substrate to a temperature ofbetween approximately 1800° F. and approximately 3200° F.; and (f)forming a diffusion coated article in which said carbon fibers remainfreely movable relative to the applied coatings by reacting said shapedsubstrate with a siliceous material for a period of time sufficient topermit the silicon to react with the pyrolytic carbon coating depositedon said fibers.
 2. A method of making a composite article as defined inclaim 1 including the step of removing said diffusion coated articlefrom said second controlled environment, returning said first controlledenvironment, and:(a) heating said article to a temperature of betweenapproximately 1800° F. and approximately 3200° F.; and (b) exposing saidarticle to a gas containing carbon and silicon selected from a groupconsisting of dimethyldichlorosilane, silicon tetrachloridetrichlorosilane, dichlorosilane silicon dichloride, silicontetrabromide, tribromosilane, dibromosilane and silicon dibromide toform a uniform chemical vapor deposition coating of silicon carbide overthe coated fibers and periphery of said difusion coated article.
 3. Amethod of making a composite article as defined in claim 1 in which saiddifusion coated article is formed by immersing said shaped substrateinto a bath of molten siliceous material for a period of time sufficientto permit the silicon to react with the pyrolytic carbon coatingdeposited on said fibers.
 4. A method of making a composite article asdefined in claim 3 in which said shaped substrate is immersed in thebath of molten silicon for a time period of between about 2 and 5minutes.
 5. A method of making a composite article as defined in claim 3in which after said shaped substrate is removed from said bath it issuspended thereabove and maintained at a temperature of on the order of2700° F. for about two minutes.
 6. A method of making a compositearticle as defined in claim 1 in which said diffusion coated article isformed by depositing a slurry of silaceous material onto said shapedsubstrate.
 7. A method of making a composite article as defined in claim1 in which said diffusion coated article is formed by diffusing a gascontaining silaceous material interstitially of said shaped substrate.8. A method of making a composite article as defined in claim 1 in whichsaid diffusion coated article is formed by maintaining said shapedsubstrate in the presence of a gas containing silicon at a temperaturegreater than about 1800° F. to deposit a coating of silicon over thecoated fibers and periphery of said shaped substrate.
 9. A method ofmaking a composite article as defined in claim 1 in which saidhydrocarbon gas is selected from a group consisting of methane, propane,ethane, butane, ethylene, acetylene and benzine.
 10. A method of makinga composite article as defined in claim 1 in which said multiplicity ofcarbon fibers are formed into a starting substrate having a fiber volumeof between about 5% and about 65%.
 11. A method of making a compositearticle comprising the steps of:(a) forming a starting substrate from amultiplicity of carbon fibers; (b) suspending said starting substratewithin a first vacuum pressure controlled environment; (c) forming anintermediate substrate by heating said starting substrate to atemperature of between approximately 1500° F. and approximately 4200° F.while exposing said starting substrate to a hydrocarbon gas to form auniform layer of pyrolytic carbon about each of the fibers in saidstarting substrate; (d) removing said intermediate substrate from saidfirst controlled environment and forming it into a shaped substratehaving the approximate shape desired of the end product compositearticle; (e) supporting said shaped substrate in a second controlledenvironment while heating said shaped substrate to a temperature ofbetween approximately 1800° F. and approximately 3200° F.; (f) forming adiffusion coated article in which said carbon fibers remain freelymovable relative to the applied coatings by reacting said shapedsubstrate with a siliceous material for a period of time sufficient topermit the silicon to react with the pyrolytic carbon coating depositedon said fibers; (g) removing said diffusion coated article from saidsecond controlled environment; (h) heating said article within acontrolled environment to a temperature of between approximately 1800°F. and approximately 3200° F.; and (i) exposing said article to a gascontaining carbon and silicon to form a uniform chemical vapordeposition coating of silicon carbide over the coated fibers andperiphery of said diffusion coated article.
 12. A method of making acomposite article as defined in claim 11 in which said second controlledenvironment is greater than atmospheric pressure.
 13. A method of makinga composite article as defined in claim 11 in which said multiplicity ofcarbon fibers is in the form of a felt material having a multiplicity ofrandomly oriented fibers.
 14. A method of making a composite article asdefined in claim 11 in which said multiplicity of carbon fibers is inthe form of a macerated material having a multiplicity of randomlyoriented fibers.
 15. A method of making a composite article as definedin claim 11 in which said multiplicity of carbon fibers is in the formof a macerated material having a multiplicity of chopped fibers.
 16. Amethod of making a composite article as defined in claim 11 in whichsaid multiplicity of carbon fibers is in the form of a woven materialhaving interwoven carbon fibers.
 17. A method of making a compositearticle as defined in claim 11 in which said multiplicity of carbonfibers is in the form of a tape material having interwoven carbonfibers.
 18. A method of making a composite article comprising the stepsof:(a) forming a starting substrate from a multiplicity of carbon fibersin the form of a woven material having a multiplicity of interwovenfibers said fibers being selected from a group consisting of pyrolyzedwool, rayon, polyacrylonitrile and pitch fibers; (b) suspending saidstarting substrate within a first controlled environment; (c) forming anintermediate substrate by heating said starting substrate to atemperature of between approximately 1500° F. and approximately 4200° F.while exposing said starting substrate to a hydrocarbon gas selectedfrom a group consisting of methane, propane, ethane, butane, ethylene,acetylene and benzine to form a uniform layer of pyrolytic carbon abouteach of the fibers in said starting substrate; (d) removing saidintermediate substrate from said first controlled environment andforming it into a shaped substrate having the approximate shape desiredof the end product composite article; (e) supporting said shapedsubstrate in a second controlled environment at less than atmosphericpressure while heating said shaped substrate to a temperature of betweenapproximately 1800° F. and approximately 3200° F.; (f) forming adiffusion coated article in which said carbon fibers remain freelymovable relative to the applied coatings by immersing said shapedsubstrate into a bath of molten siliceous material for a period ofbetween about 2 to 5 minutes to permit the silicon to react with thepyrolytic carbon coating deposited on said fibers; (g) removing saidarticle from said bath and suspending it thereabove while maintainingsame at a temperature of on the order of 3200° F. for about two minutes;(h) removing said diffusion coated article from said second controlledenvironment; (i) heating said article within a controlled environment toa temperature of between approximately 1800° F. and approximately 3200°F.; and (j) exposing said article to a gas containing carbon and siliconselected from a group consisting of dimethyldichlorosilane, silicontetrachloride trichlorosilane, dichlorosilane silicon dichloride,silicon tetrabromide, tribromosilane, dibromosilane and silicondibromide to form a uniform chemical vapor deposition coating of siliconcarbide over the coated fibers and periphery of said diffusion coatedarticle.